 Following law school, Southwick clerked for the Presiding Judge, John F. Onion, Jr., of the Texas Court of Criminal Appeals from 1975 to 1976, and then, in Mississippi, for Judge Charles Clark of the United States Fifth Circuit Court of Appeals from 1976 to 1977.Southwick was in private practice as an attorney in Jackson, Mississippi with the firm Brunini, Grantham, Grower & Hewes from 1977 to 1989, serving as a partner from 1983 to 1989.In 1989, Southwick entered government service as a Deputy Assistant Attorney General for the United States Department of Justice Civil Division.